Conventional sweep circuits used for horizontal and vertical scanning in television receivers periodically emit a blanking pulse, derived from the sawtooth voltage of a respective ramp generator, in order to suppress the beam during its return stroke.
If, for any reason, the vertical-sweep generator fails to operate properly so that the beam oscillates along a single horizontal line of the CRT screen, the sensitive layer of that screen may be irreparably damaged unless the beam is promptly extinguished. It has therefore already been proposed to provide protective circuitry for suppressing the beam whenever a flyback pulse, normally developed across the vertical-deflection yoke of the CRT within the blanking period, fails to materialize. Such a system generally comprises a storage capacitor chargeable by a guard circuit in response to the flyback pulse; in the discharged state of the capacitor, the blanking pulse is extended into a continuous d-c voltage whereby the beam is permanently cut off. See, for example, Technical Note 078 by N. V. Philips'Gloeilampenfabrieken (1978) relating to the integrated vertical-deflection module known as TDA2652.
The storage capacitor used with such a prior-art guard circuit must be fairly large, with a capacitance of at least one microfarad. Such capacitors cannot be incorporated in an integrated-circuit module and are therefore designed as separate circuit components connected to the i-c chip via an external terminal.